^inirrira 



n Fis/irrirs Socirli/ 329 



C()iH|)aris()ii to tlie mass of tlio oru-anisms \\-1iicli draw tlK'ir 

 o.w.a-cn from it, is relatively far greater than the mass of tlu' 

 Idood with refereiiee to that of the eells which take tlieir oxv.u'eii 

 from it. \vt is it none the less true that the supply of oxvovn 

 ill most lakes is vei'v small as coiuparcHl with that of an aiiiinal. 

 aii<l the nieehanisiu for reiu'wirii;- it is alwavs \-erv ineltieieiit as 

 (•oni|»are(l with the deiiiand for tlu' ,u'as. 



The disadxaiitau'e of the lake in the matter of respiration ap- 

 pears still more clearly when we consider the means (d' trans- 

 portin_ii- the oxyacn from the reuioii wlu're it is absorbed — the 

 surface — to the deei»er parts of the lake, wheiv much of it is to he 

 used. The animal shows a comi)iex and very efticient mechanism 

 for the circnlatioii of the Mood; an api)aratus whose conii)l 'xity 

 and efticiency ai-e in lar,L;'e measure deteriiiiiie(l hy the necessity 

 for a rapid distribution of the oxy.u'en and a rapid disi)osal of the 

 ,U-aseous wastes of the' body. In the lake the means of transport 

 are tlnx'i': diffusiou. by which the o-as is slowly jiassed from point 

 to jioiut iu the watei' independently of the ciirreius: currents 

 produced by the wind : and convection currents. i)ro(luced by tlu' 

 coolin*^- of the surface water to a tem])erature below that of the 

 water be'iieath. 



Diffusion is a i)roccss Avhich o})erates ra})i(lly when the dis- 

 tances are minute, hiit whose efficiency decreases ,uTeatly as the 

 distances increase. !n our hin.u's, or the .u'ills of a tish, for in- 

 stance, where thi' distance betwet'ii blood and air is measured 

 in thousandths of an inch the ))r()cess of diffusion j^oes on with 

 great rapidity. But where, as in the lake, the distanees are 

 measured by inches or by feet, or even l)y scores of feet, the 

 ])roct'ss is [)ractically worthless for the jti-ocesses of distril)ution. 

 By diffusion alone oxygen would penetrate the lake only to the 

 dejjth of a very few feet in a whole season. While diffusion, 

 therefore, plays an active and important ]iart in the exchange of 

 gases l)etween the indi\idual plant and animal and the water 

 immediately surrounding it. if has little or nothing to do with 

 the general circulaticni of gases within the lake. 



During the fall, when the lake is cooling, convection cur- 

 rents aid materially in carrying oxygen down to considerable 

 depths. The surface water, saturated with oxygen, cools, be- 

 comes heavier, and sinks, carrving the gas with it. The same 



